Numerous applications of precision optical components require reflector elements with a mirror layer that is highly reflective over a broad spectral range, for example, for applications in astronomy or in space optics (for example, for observation of the Earth). Metals show high reflectance over a broad spectrum. Depending on the spectral range, gold, aluminum, or silver are generally used for reflectors.
Gold possesses highly favorable chemical resistance and high reflectance in the IR range. Aluminum shows high reflectance up to the UV range. Of all metals, silver shows the highest reflectance from the VIS- up to the IR range. Like electrical conductivity, reflectance is also dependent on the number of defects in the respective metal (Drude theory). In order to achieve high targeted reflectance, a metal layer that is as defect-free and smooth as possible is necessary. Solutions for reflectors are also known in which metals are combined. For special applications, a targeted desired ratio of transmitted to reflected light can be set (beam splitter). Thin metal layers can be used for this purpose.
In reflectors requiring a maximum reflectance over a limited spectral bandwidth and angle of incidence range, purely dielectric layers (without a reflecting metal layer) can be used to adjust the reflectance. In these reflectors, the interference effect is used. A plurality of layers of dielectric materials having differing refractive indices are combined.
In both reflectors having a dielectric layer system and reflectors based on the reflectance of metals, aging effects and defects may occur. In metals in particular, there is the risk of corrosion. In order to prevent this, the metal layers are often equipped with a protective layer.
While applications without applied protective layers exist for aluminum and gold, silver reflectors are virtually always provided with a protective layer. The production of protected reflectors in front side mirrors that must show maximum reflectivity over a broad spectral range is particularly demanding, as the protective layers affect the optical function of the layers. Only certain dielectric materials can be used for the protective layer in order to keep the negative effect on optical performance as low as possible. A targeted increase in reflecting in certain spectral areas can be achieved by utilizing the interference effect through a combination of different dielectric materials as a protective layer.
For example, for protection and increasing the reflectance of metallic reflectors, protective layers can be deposited on the reflecting metal by means of PVD, CVD or ALD. By means of this method, the dielectric materials in question can be deposited on the reflectors with precisely defined layer thicknesses. Deposition parameters with low process temperatures (generally T<150° C.) are selected. As a rule, subsequent thermal treatments at T>150° C. are dispensed with. High temperatures are avoided in order to prevent warping of the reflector. For example, different thermal expansion coefficients of the layer and substrate in combination with major temperature fluctuations can lead to warping.
In order to achieve precise guidance in an optical instrument, no warping can be allowed, or high dimensional accuracy of the reflector must be preserved. In addition, many substrates are temperature-sensitive. High temperatures or rapid temperature changes would damage such substrates.
From other standpoints, however, high temperatures can lead to an improvement of the reflector. For example, deposition at the lowest possible temperature allows the dielectric layers to be porous. In this state, some materials form layers that are not moisture-tight (moisture can pass through the layer to the metal layer to be protected), or they are readily soluble (in a moist environment, the layer is dissolved and thus decomposes over time). Conversion to a more stable state would be possible by means of thermal treatment, for example, in a furnace. In addition, such thermal treatment or deposition at high temperature could reduce the absorption of transparent layers or improve the reflectance of metal layers. However, this is impossible or possible only to a very limited extent because of the resulting temperature effect on the substrate and partially also on the metal layer to be protected.